1. Field of the Invention
This invention relates to a photopolymerizable element for use as a flexographic printing plate and a process for preparing the plate from the element. The photopolymerizable element has at least one photopolymerizable elastomeric layer that comprises a binder, a monomer, a photoinitiator, an onium salt, and a leuco dye.
2. Description of Related Art
Flexographic printing plates are widely used for printing of packaging materials ranging from corrugated carton boxes to cardboard boxes and to continuous web of plastic films. Flexographic printing plates are used in relief printing in which ink is carried from a raised-image surface and transferred to a substrate. Flexographic printing plates can be prepared from photopolymerizable compositions, such as those described in U.S. Pat. Nos. 4,323,637 and 4,427,759. The photopolymerizable compositions generally comprise an elastomeric binder, at least one monomer and a photoinitiator. Photosensitive elements generally have a solid layer of the photopolymerizable composition interposed between a support and a coversheet or a multilayer cover element. Flexographic printing plates are characterized by their ability to crosslink or cure upon exposure to actinic radiation. Typically, the plate is uniformly exposed through its backside, i.e., backflashed, to a specified amount of actinic radiation to form a floor. Next, the plate is imagewise exposed through its front side with the same actinic radiation that was used for the backflash exposure. The imagewise exposure is made through an image-bearing art-work or a template, such as a photographic negative or transparency (e.g. silver halide films), or through an in-situ mask having radiation opaque areas that had been previously formed above the photopolymerizable layer. The actinic radiation exposures can be conducted with ultraviolet (UV) or black light. The actinic radiation enters the photosensitive element through the clear areas and is blocked from entering the black or opaque areas of the tranparency or in-situ mask. The areas of the photopolymerizable layer that were exposed to the actinic radiation crosslink and harden and/or become insoluble to solvents used during development. The unexposed areas of the photopolymerizable layer that were under the opaque regions of the transparency or in-situ mask during exposure do not hardened and/or remain soluble. The unexposed areas are removed by treating with washout solutions or heat leaving a relief image suitable for printing. If treated with washout solutions, the plate is dried. The printing plate can be further exposed to complete polymerization and to remove surface tackiness. After all desired processing steps, the plate is then mounted on a cylinder and used for printing.
The above method for forming a flexographic printing plate from such a photosensitive element can be facilitated at the various steps described above by the ability to see the image created in the photopolymerizable layer. Typically the image cannot be easily seen in the photopolymerizable layer prior to forming the relief, and is in effect a latent image. A change in color in exposed areas of the photopolymerizable layer as a result of the imagewise exposure to actinic radiation would allow for easy visual observation of the image. One could readily determine at a glance if a plate has been imagewise exposed or not. Also, contrasting colors would easily distinguish the unexposed areas from the exposed areas. The plate having a color contrasting image could be preliminarily checked for the desired image and image quality prior to treating. The ability to check if a plate has been exposed and if it has the desired image is particularly useful in plate making shops where many plates are handled and may expose several plates prior to treating.
A change in color in the photopolymerizable layer as a result of imagewise exposure can also be an indicator that the photopolymerizable layer has been given desired imagewise exposure to form the relief. Too long of exposure of the element will overharden the polymerizable layer and ultimately cause cracking in the printing surface of the plate. If the plate is underexposed, the plate will not have desired relief necessary for flexographic printing.
Further, it would be desirable to maintain color contrast of the image after imagewise exposure and the treating step, as well as the subsequent exposure steps, to form the relief printing plate. It is advantageous for a flexographic printing plate to have contrasting color between the exposed (raised) areas and the floor (recessed) areas of the image. Color contrast of the print-ready flexographic printing plate is helpful for mounting of the plate on printing cylinder, particularly on printing presses having multiple stations where plates need to be mounted in register on multiple printing cylinders.
It is known to include dyes, particularly leuco dyes, in photopolymeric radiation curable systems to produce colored images in the exposed regions of the photopolymer. See “Photopolymers: Radiation Curable Imaging Systems,” B. M. Monroe, in Radiation Curing: Science and Technology, Pappas, S. P., ed., Plenum Press, NewYork (1992). In this case, the photoinitiator system generally provides the free radicals necessary to convert the leuco dye to its color form. The leuco dye must then compete with the monomer/s for the free radicals in the photopolymerizable composition. A problem with this approach to provide color contrast, particularly for the elastomeric, relatively thick, photopolymerizable layer/s associated with flexographic printing elements, is that the leuco dye will not convert to its color form in the exposure time necessary to polymerize the exposed areas of the photopolymerizable layer.
U.S. Pat. No. 5,942,554 discloses a method for the formation of highly colored polymeric bodies which includes subjecting a curable composition containing a color precursor and an onium salt to heat or actinic radiation to cure the composition where the color precursor is converted to its colored form. The method provides for simultaneous color development and photopolymerization of curable films using a color precursor such as a lactone and an onium salt such as iodonium or sulfonium salt. In addition to the color precursor and the onuim salt, the curable composition includes curable compounds, such as free-radical curable and/or cation curable monomers. A photoinitator may optionally be present in the curable composition. However, only thin films up to 15 micron (0.59 mils) of the curable composition were disclosed. Prior to polymerization, the thin film is a liquid layer. There is no suggestion that the method is useful in a flexographic printing element having a photopolymerizable layer which is thick, that is greater than 20 mils (>508 micron), and is a solid elastomeric layer.